[neilep (OP)]

Dear Eggsperts (again)

Why do birds/reptiles/some fish etc lay eggs ?

Hmmmmmmmmm....I think I may know the answer for birdies....because they would not be able to fly with a  load of birdies inside them during gestation....but then why do crocs lay eggs ?...is my reasoning about birds correct ?


Why don't we lay eggs and stick them in the airing cupboard ?

[dentstudent]

Is it because much more of the embryonic growth occurs outside the mothers body? This means that many more offspring can be borne to one mother at once, or on successive days, as there is very little development inside the parent. Non egg layers tend to have fewer babies.

Not very well put, but do you see my point?

[Karen W.]

Is it because much more of the embryonic growth occurs outside the mothers body? This means that many more offspring can be borne to one mother at once, or on successive days, as there is very little development inside the parent. Non egg layers tend to have fewer babies.

Not very well put, but do you see my point?

It is mighty uncomfortable for a human to carry 6 0r 8 children.. and imagine a tiny bird like you said. OHH another topic!

[neilep (OP)]

Is it because much more of the embryonic growth occurs outside the mothers body? This means that many more offspring can be borne to one mother at once, or on successive days, as there is very little development inside the parent. Non egg layers tend to have fewer babies.

Not very well put, but do you see my point?

I do see your point and I am grateful for it Stu,........but.......the birdies don;t grow any larger than the eggs they are in....you know what I mean ! ?..........oh hang on !!!  (Slaps own face)....it's not as if birdy has all 4-6 eggs in her at the same time is it ...?...DOH !!!

See ?.....you is clevur....me is knot !!

THANKS STU.......

.......However...crocs are like...well big !!....but they do lay eggs in vast quantity don't they ?...

..hmmmm....ok...end of thread !!!

[dkv]

In order to evolve towards greater pleasure.
Some ciritical body parts need greater care.
For exmaple: proper growth of nervous system.
FOR sustainable pleasure - Mind is also required..
These organs can not develop without proper care ...
Greater the complexity more time it takes to reproduce and greater the ability to experience.

Egg laying animals are less evolved .. their ability to experience is low.. that why we eat chicken more frequently.
However we need to understand importance of certain other animals at least at an individual level.
These animals are Dolphins , Elephants and most of the complex mammals... there ability to experience is greater .. they are emotional and it is a scientific fact.

[Andrew K Fletcher]

Nope, there are lots of non-egg layers that have lots of babies, dogs who can have 14 puppies and cats for example. Anyway laying eggs is not a decision it's a trait for each species, so the question as to why some creatures lay eggs has not been answered, so the answer must be in the development of a shell around the fertilised eggs. We know that poor diet and disease can cause poor shell development so maybe the answer lies in we are what we eat?

[another_someone]

Dogs and cats may have multiple offspring in a single litter, but none will reach in their millions.  By no means is it the case that all egg laying species will lay millions of eggs, but some certainly do.

The interesting questions pertain to species that lay eggs, but then look after the eggs until they hatch (e.g. most birds).  Clearly, in species that have minimum investment in each egg, and simply lay very large numbers of fertilised eggs on the assumption that there will be a high mortality rate amongst the eggs, but despite that, a significant number will survive; they are clearly utilising a different, but equally legitimate, survival strategy to that where an animal will place a higher investment in a fewer number of fertilised eggs.

In the case of birds, one would assume that not having to carry live young will reduce their weight, which is important for flight - except that this is an issue that does not stop bats from bearing live young, and yet have an airborne life style.  So in these cases, one must assume that both options have equal value, and it is merely a matter of the history of the species that dictates which is the solution that is followed by the species.

[SquarishTriangle]

Let's visit our old friend the phylogenetic tree...(well, Linnaeus is 300 years old this year!!)...

In extremely simplified terms, the birds, turtles, crocodilians, amphibians and mammals are all derived from a common ancestor that was an egg layer. That ancestor can be traced back to the fishes, also egg layers. The sexually-reproducing invertebrates (from which the vertebrates descended) also produced eggs.

External fertilisation can be considered the ancestral characteristic and results in the formation of an egg that develops outside the body. From there, variations on the common theme have arisen, including animals that fertilise internally but still develop their eggs externally. Only in the mammals (and some of the sharks) has the foetus being retained within the mother's body and nurtured there until birth.

When the first aquatic tetrapod (a salamander-like animal) ventured onto land to become our terrestrial ancestor, a number of terrestrial adaptations were required for it to survive and reproduce successfully. One, as mentioned, was that to internal fertilisation (because there is obviously much less water on land to act as a medium for sperm to meet egg than in the ocean). Another was that the egg laid had to be protected in some way from drying out (in the form of a leathery membrane, or shell).

The benefits of laying an egg and having it developed outside the body include, as was mentioned, not having to carry around the extra weight. But it also comes at the energitic cost of having to lay the egg with a good supply of yolk and a protective shell. Contrarily, mammals have opted to carry the extra load (which is handy if you need to move around a bit to forage, rather than constantly guarding your clutch eggs)...probably finding this trait sufficiently benefial to their survival to outweigh the inconvenience of gestation.

But here's the REALLY important bit...if you start at the very first animal to reproduce sexually (ie produce an egg) in the phylogenetic tree, and trace it all the way up to birds you'd have go through numerous taxa (probably all of which produce eggs) and many millions of years. The point:

The egg CLEARLY had to come before the chicken!

[chris]

Excellent answer. Thanks

[SquarishTriangle]

Awww thank you Chris.

[Alandriel]

Superb Squarish triangle! Thanks for that !!

Egg before the chicken - one of the univers' mystery FINALLY solved  []

[neilep (OP)]

With all these FANTASTIC answers I am truly bettered !!

THANK YOU very much !!

[SquarishTriangle]

That's a neat little diagram. You wouldn't have been impressed with what I was thinking of putting up there.

[another_someone]

Only in the mammals (and some of the sharks) has the foetus being retained within the mother's body and nurtured there until birth.

I think the number of animals that produce live young are slightly more than that, although certainly mammals are the only group for which this is exclusively so.

I believe that tuna (or some tuna) may produce live young (again, tuna, like sharks, because of their physical size, are bordering on being warm blooded, and so would probably need to keep their eggs warm - birds too need to do this, but they do it while having the eggs external, and sitting on them).

I believe that some snakes may also give birth to live young, although not sure which ones they are.

No doubt, there are some other species - I do not claim that the list is exhaustive - but only to show that even amongst non-mammals, there are quite a few (even if still very much a minority) or species that do give birth to live young.

[SquarishTriangle]

Now that you mention it, live birth does seem to have arisen independently several times in the course of evolution.

The majority of snakes, depending on species, are oviparous (egg laying), but others can be ovoviviparous (giving birth to live young nourished by yolk) or viviparous (giving birth to live young nourished by a placenta). Ovoviviparity can be looked at as an intermediate between egg laying and viviparity. In the case of such animals, the young hatch from their eggs while still inside the mother’s body, prior to emerging to the outside world. Sharks and rays also all fertilise internally and again have the three modes of development.

(Are you still reading this?)

I should also point out that not all mammals are viviparous. The Monotremes (subclass), consisting of the platypus and two species of echidna, branched off before viviparity arose in the remaining mammals, the Therians (Eutherian or ‘placental’ mammals, and the marsupials). Instead they lay eggs (but still produce milk for their young). The marsupials are interesting in that they give birth to live, but extremely immature young. Instead of investing significant amounts of time and energy in gestation, a tiny offspring (of jellybean proportions) emerges and crawls its way blind through the mother’s fur to her pouch, where it is nourished for an extensive period on the lactating teat.

In a more peculiar version of internal development, the female gastric brooding frog (assumed extinct since the 1980s) would swallow her clutch of eggs after laying them, and incubate them in her stomach. The young would hatch as tadpoles and develop there until they were ready to emerge as young froglets. The secretion of digestive acid would cease during this period.

Hmm…are you sure about the live birth for tuna? I did think they spawn. But they never cease to amaze...

[paul.fr]

...for us to eat!

[SquarishTriangle]

...because, of course, if our attention turns to the eggs, the animal may then have a better chance of avoiding our dinner plates. They may even gain protection, housing and food from humans in want of some of their ovarian goodness. Add that to the natural survival mechanism of hatchings...irresistable cuteness...which ensures that not even a big, mean hairy human would dare harm them...and you end up with an egg-laying creature with a distinct evolutionary advantage.

That's what you meant right, Paul?

[another_someone]

Hmm…are you sure about the live birth for tuna? I did think they spawn. But they never cease to amaze...

Sorry, my mistake - looking at it again, the only references I can find are to egg laying tuna, and cannot find any contrary references.